Metamorphic Rocks for Kids | Origin and Classification

‘Metamorphism’ comes from joining two Greek words i.e., meta meaning ‘change’ and morpho refers to ‘shape’. It means metamorphism means ‘to change shape’. Metamorphism is a process in which high temperature and pressure changes the mineral content of rocks through metamorphic reactions such that the parent (original) rock bears no resemblance at all with the newly transformed rock. Thus, not only chemical composition of rocks is changed but texture as well. This process only takes place in solid rocks.

Metamorphic rocks are derived from igneous and sedimentary rocks. As soon as rock experiences a change in its physical and chemical conditions, it undergoes metamorphism. During chemical reactions, minerals inside the rock are changed so much that the entire texture of the rock alters with it. The higher the temperature, the faster will be the changes caused by metamorphic reactions.

The interior of the earth gets hotter as depth increases. Metamorphism can take place even at a temperature as low as 50oC (temperature of hot cup of coffee). However in such a case, metamorphic reactions require millions and millions of years to occur. The maximum temperature at which metamorphism can take place varies but it generally lies somewhere between 600oC – 1200oC.

Agents of Metamorphism

Agents are those factors of metamorphism that set this process in motion. The four main agents are:

Temperature (heat)

Pressure

Fluid activity

Time

Grades of Metamorphism

The intensity of metamorphism is called metamorphic grade. In other words, metamorphic grades tell us how much a rock is changed with respect to changes caused by metamorphism. The grades of metamorphism rise with increase in depth.

High-grade metamorphism – change of mineral composition at high temperatures and pressures

Metamorphic Changes

Metamorphism causes two kinds of changes to a rock i.e.,

Changes in texture

Changes in mineral content

Changes in texture

The shapes as well as sizes of grains are changed during metamorphism. Some grains get larger while others smaller.

Changes in mineral content

If there is only one mineral in the parent rock, metamorphism will not give rise to a new mineral. Its texture will turn into uneven but metamorphism will not cause any change in mineral content. This is due to lack of additional chemicals (no chemicals are added in metamorphism).

Example

Limestone → marble

Then again if there are more than one mineral in the parent rock, metamorphism will transform those minerals into new kinds along with different texture. It happens when rising temperature allows few minerals to decompose and in this way, their atoms reorganize with other minerals to form new ones.

Metamorphism occurs as a result of change in physical and chemical conditions of a rock. During metamorphic reactions, the minerals inside the rock are changed and replaced by a new mineral, which is stronger than the previous one to endure high temperature. This kind of reaction is known as heterogeneous metamorphic reaction.

It is however not necessary that a new mineral is formed in the metamorphism of every rock. At times, the composition of minerals does not change instead crystals inside the rock starts maturing again. This process is known as recrystallization.

In some cases, atoms inside the minerals respond to metamorphic reactions. The atoms are reorganized in such a way that they produce more compact structures. These reactions are known as polymorphic transformation. In recrystallization, the grains are changed in size only but polymorphic reactions also alter the way by which atoms are organized.

Examples

Limestone consists of two minerals calcite (CaCO3) and quartz (SiO2). These minerals are held together by strong ionic bonds. With increasing temperature and pressure, these bonds tend to change shape. It is because these minerals can no longer keep up with this higher temperature which is putting extra stress on the bonds. So to take pressure off, a new mineral is formed with the capacity to withstand this temperature quite easily. This mineral is called wollastonite (CaSiO3).

CaCO3 + SiO2 → CaSiO3 + CO2

“Note that temperature is not high enough to rip apart ionic bonds because if that happens, the rock will no longer exist in solid state and change into liquid”.

During metamorphic reactions of quartz sandstone, recrystallization takes place and there is no change in minerals.

A mineral olivine is found at earth’s mantle (about 600 km deep). During metamorphism, it rearranges its atomic structure and turn into a new mineral called spinel.

Types of Metamorphism

Geologists classify metamorphism into four kinds i.e.,

Contact metamorphism

Burial metamorphism

Regional metamorphism

Hydrothermal metamorphism

Contact Metamorphism

Magma is extremely hot (hotter than lava) but the surrounding rocks (called country rocks) are cooler. However, when hot magma intrudes country rocks, it leads to metamorphism which is labeled as contact metamorphism. The intensity of metamorphism (known as metamorphic grade) is highest at the point of contact (between country rocks and magma) and lowest farther away. This way the minerals inside the country rocks are changed into new ones.

The country rocks can be igneous, sedimentary or even metamorphic rocks.

This kind of metamorphism is also known as thermal metamorphism.

Mafic magmas are more effective in causing thermal alteration than felsic magmas.

Example

Shale →[contact metamorphism]→ hornfels

Burial Metamorphism

As you know that running water transports and deposits heavy sediment into the oceans. This sediment is deposited in layers. After tens of millions of years, sediment deposition becomes extremely intense and heavy. Eventually the water could no longer withstand the weight of sedimentary basin and go under the surface. Thus, rocks are buried inside and the newer segments of sediment push the older rocks to go deeper. As years go by and rocks go deeper, both temperature and pressure increase leading to metamorphism which is called burial metamorphism.

Illustration

A boat sinks if too many people leap on it.

Example

Limestone/dolomite →[burial metamorphism]→ marble

Regional Metamorphism

It is the most familiar kind of metamorphism that takes place near the place of subduction zone. A subduction zone is a region where tectonic plates glide over one another such that one plate goes below the other, in the earth’s mantle. A rising magma increases the temperature of the country rocks which in turn changes the mineral composition of these rocks. Besides, tectonic forces (together with hot magma) also play their part by compressing these rocks and give them new shape.

Regional metamorphism

Example

Quartz sandstone →[regional metamorphism]→ foliated quartzite

Limestone →[regional metamorphism]→ foliated marble

Hydrothermal Metamorphism

Water is an active fluid. The dissolving capacity of water increases as its temperature goes up. Now the ions present in this hot water alter minerals as well as chemical composition of the rock when it comes into contact with it. This metamorphism is known as hydrothermal metamorphism.

It is also known as hydrothermal alteration.

Importance of Metamorphic Rocks

Marble is a metamorphic rock. It has been widely used in making of sculptors, prehistoric buildings and monuments like Taj Mahal (India), pyramids in olden times etc. Some of the recent monuments are Lincoln Memorial, the Washington Monument and the like.

Slate is also a metamorphic rock with immense applications. It is used in alleyways, flooring as well as making of blackboards and the like.

The Washington Monument standing 169 meters tall is built of marble. It is the tallest structure in the world which is made of stone. The structure is built from three different kinds of marble. There is a clear difference of color in all three.

Metamorphism and Global Warming

Metamorphism occurs on all plate boundaries. As it happens, a lot of carbon dioxide gas along with water is given off into the air. CO2 is a greenhouse gas that traps solar radiation after striking from earth’s surface and keeps the atmosphere warm. Thus it is quite possible that metamorphism increases global warming.

The time period between 56 and 34 million years ago is known as Eocene. Today, the atmosphere on earth is not as warm as during the times of Eocene when the concentration of CO2 varied between 700 to 2000 ppm (parts per million). However at present, CO2 is only about 380 ppm. In Eocene, high concentration of CO2 was perhaps on the basis of metamorphism which took place all over the world as India crashed into the continent of Asia and Himalayan Mountains stood up as a result.